stm32/mboot: Leave bootloader from thread mode, not from IRQ.

Leaving the bootloader from an IRQ (eg USB or I2C IRQ) will not work if
MBOOT_LEAVE_BOOTLOADER_VIA_RESET is disabled, ie if mboot jumps directly to
the application.  This is because the CPU will still be in IRQ state when
the application starts and IRQs of lower priority will be blocked.

Fix this by setting a flag when the bootloader should finish, and exit the
bootloader always from the main (top level) thread.

This also improves the USB behaviour of mboot: it no longer abruptly
disconnects when the manifest command is sent.

Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/mboot/dfu.h

@@ -26,6 +26,7 @@
 #ifndef MICROPY_INCLUDED_STM32_MBOOT_DFU_H
 #define MICROPY_INCLUDED_STM32_MBOOT_DFU_H
 
+#include <stdbool.h>
 #include <stdint.h>
 
 // DFU spec: https://www.usb.org/sites/default/files/DFU_1.1.pdf
@@ -106,6 +107,7 @@ typedef struct _dfu_state_t {
     dfu_cmd_t cmd;
     dfu_status_t status;
     uint8_t error;
+    bool leave_dfu;
     uint16_t wBlockNum;
     uint16_t wLength;
     uint32_t addr;

ports/stm32/mboot/main.c

@@ -106,8 +106,6 @@ static volatile uint32_t systick_ms;
 // Global dfu state
 dfu_context_t dfu_context SECTION_NOZERO_BSS;
 
-static void do_reset(void);
-
 uint32_t get_le32(const uint8_t *b) {
     return b[0] | b[1] << 8 | b[2] << 16 | b[3] << 24;
 }
@@ -771,7 +769,7 @@ void i2c_slave_process_rx_end(i2c_slave_t *i2c) {
         memcpy(buf + 12 + sizeof(MICROPY_HW_MCU_NAME), MICROPY_HW_BOARD_NAME, sizeof(MICROPY_HW_BOARD_NAME) - 1);
         len = 12 + sizeof(MICROPY_HW_MCU_NAME) + sizeof(MICROPY_HW_BOARD_NAME) - 1;
     } else if (buf[0] == I2C_CMD_RESET && len == 0) {
-        do_reset();
+        dfu_context.leave_dfu = true;
     } else if (buf[0] == I2C_CMD_GETLAYOUT && len == 0) {
         len = strlen(FLASH_LAYOUT_STR);
         memcpy(buf, FLASH_LAYOUT_STR, len);
@@ -864,6 +862,7 @@ static void dfu_init(void) {
     dfu_context.cmd = DFU_CMD_NONE;
     dfu_context.status = DFU_STATUS_OK;
     dfu_context.error = 0;
+    dfu_context.leave_dfu = false;
     dfu_context.addr = 0x08000000;
 }
 
@@ -931,7 +930,8 @@ static void dfu_handle_rx(int cmd, int arg, int len, const void *buf) {
 
 static void dfu_process(void) {
     if (dfu_context.state == DFU_STATE_MANIFEST) {
-        do_reset();
+        // Set a flag to leave DFU mode from the main thread (here we are in an IRQ handler).
+        dfu_context.leave_dfu = true;
     }
 
     if (dfu_context.state == DFU_STATE_BUSY) {
@@ -1412,17 +1412,6 @@ static void leave_bootloader(void) {
     NVIC_SystemReset();
 }
 
-static void do_reset(void) {
-    led_state_all(0);
-    mp_hal_delay_ms(50);
-    pyb_usbdd_shutdown();
-    #if defined(MBOOT_I2C_SCL)
-    i2c_slave_shutdown(MBOOT_I2Cx, I2Cx_EV_IRQn);
-    #endif
-    mp_hal_delay_ms(50);
-    leave_bootloader();
-}
-
 extern PCD_HandleTypeDef pcd_fs_handle;
 extern PCD_HandleTypeDef pcd_hs_handle;
 
@@ -1561,7 +1550,7 @@ void stm32_main(int initial_r0) {
     #if MBOOT_USB_RESET_ON_DISCONNECT
     bool has_connected = false;
     #endif
-    for (;;) {
+    while (!dfu_context.leave_dfu) {
         #if USE_USB_POLLING
         #if MBOOT_USB_AUTODETECT_PORT || MICROPY_HW_USB_MAIN_DEV == USB_PHY_FS_ID
         if (USB_OTG_FS->GINTSTS & USB_OTG_FS->GINTMSK) {
@@ -1585,10 +1574,20 @@ void stm32_main(int initial_r0) {
             has_connected = true;
         }
         if (has_connected && pyb_usbdd.hUSBDDevice.dev_state == USBD_STATE_SUSPENDED) {
-            do_reset();
+            break;
         }
         #endif
     }
+
+    // Shutdown and leave the bootloader.
+    led_state_all(0);
+    mp_hal_delay_ms(50);
+    pyb_usbdd_shutdown();
+    #if defined(MBOOT_I2C_SCL)
+    i2c_slave_shutdown(MBOOT_I2Cx, I2Cx_EV_IRQn);
+    #endif
+    mp_hal_delay_ms(50);
+    leave_bootloader();
 }
 
 void NMI_Handler(void) {